Magnetic Resonance Tumour Regression Grade (mrTRG) as a Novel Biomarker - Phase III Non CTIMP Trial

Open to patients undergoing any pre-operative treatment for locally advanced rectal cancer, TRIGGER is the only phase III clinical trial in the UK offering watch and wait. All patients will have post treatment MRI scans routinely performed, no change from the MERCURY trials high resolution MRI protocol is required. Patients will be randomised to either the control arm for management according to national guidelines - conventional MDT, clinical assessment post-treatment planning using the baseline MRI. Patients in the interventional arm will have their post treatment MRI scans read by a radiologist trained and supported to reliably report the mrTRG grade and have their management directed accordingly - 'Good response' (mrTRG 1\&2) - watch and wait (avoidance of surgery) offered. 'Poor response' (mrTRG 3-5) - local colorectal MDT is informed and uses information to discuss and agree next steps in treatment and surveillance. Patients are followed up for five years with QoL questionnaires completed at registration, 3 and 5 years.

The only phase III clinical trial in the UK offering watch and wait, the TRIGGER trial aims to validate mrTRG as an imaging biomarker for the stratified management of patients with locally advanced rectal cancer. The 'good responders' (mrTRG1\&2) often have no evidence of tumour and it may be possible to avoid surgery in this group and so maintaining QoL while not impacting survival rates. The 'poor responders' (mrTRG3-5) are at high risk of poor oncological outcomes and this knowledge is useful in planning ongoing treatment and surveillance. TRIGGER is now a non-cTIMP trial as the protocol does not specify chemotherapy or IMP treatments. Decisions about the use of chemotherapy will be based upon local MDT discussions as is normal practice and national policy and the trial CRFs will capture these decisions and whether more treatment is given to patients or not. TRIGGER does not mandate or recommend the use of any treatments: specifically it does not suggest the use of investigational medicinal products. If any centre wishes to use IMPs this would be in the context of separate trial protocols and would not preclude entry into TRIGGER.